RF data communication systems using baseband frequency shift keying (FSK) often employ a frequency on either side of the center frequency to represent data symbols. Each frequency is associated with a unique symbol. The symbols are represented as either a "1"or "0", and the designation of each symbol to either side of the center frequency is referred to as the channel polarity. Data communication is accomplished in such a system by transmitting frequencies in the FSK format and then decoding the symbol represented by each frequency. Unfortunately, such decoding is often unsuccessful due to channel polarity inversions.
Channel polarity inversions occur primarily as a result of the particular radio design employed. The problem is especially common where several types of radios are used on the same system or where one type of radio is used on more than one system. The system transmitter may invert the channel polarity in the transmitter preprocessing or modulating stages, while another radio may or may not be designed to invert the channel polarity in its receiver circuitry. The reasons for inverting or for not inverting are commonly due to the modulation designs, the local oscillator (L.O.) side selected for injection, and the manner in which the analog processing circuitry is designed in the discriminator portion of the radio receiver. In any case, the receiving radio may receive inverted or noninverted data and may invert or not invert the received data depending on the particular radio design.
In the past, such inversions have been compensated for by programming each radio with inversion bits for each system the radio may operate on. The radio may then employ such information to determine whether or not to invert the data received from the associated system. Unfortunately, this technique, requiring a prior knowledge of the system characteristics, has caused a number of problems in the manufacturing and the field maintenance of the radios.
In manufacturing, this technique has been a problem because the inversion bits must first be programmed to test the radio, and once again to set up the radio for the system(s) it will operate on. Although the efforts required to effect such programming may be minimal, it is well recognized that human interface increases the potential for error. In situations where the inversion bits are programmed incorrectly, the radio is inoperable until serviced and programmed correctly.
In field maintenance, a similar problem results. Not only must the technician servicing the radio be knowledgable about the programming requisites for the radio to repair it, but knowledge of the programming requisites for each system must also be available in order to correctly (re)set the inversion bits. The maintenance and distribution of such information is extremely burdensome.
Accordingly, a radio design is needed which can overcome the foregoing deficiencies.